As is well known to those skilled in the art, various processes and catalysts exist for the homopolymerization or copolymerization of .alpha.-olefins. For example, processes are known for polymerizing ethylene or propylene, either alone or in the presence of small quantities of other monomers, to produce plastics. These plastics are typically used in such applications as blow and injection molding, extrusion coating, film and sheeting, pipe, wire and cable. Also for example, it is well known to copolymerize ethylene, propylene, and optionally a third monomer such as non-conjugated dienes, to make elastomers. Ethylene-propylene elastomers find many end-use applications due to their resistance to weather, good heat aging properties and their ability to be compounded with large quantities of fillers and plasticizers. Typical automotive uses are radiator and heater hose, vacuum tubing, weather stripping and sponge doorseals. Typical industrial uses are for sponge parts, gaskets and seals.
Due to their different properties and end uses, it is important to distinguish between those factors affecting elastomeric or plastic properties of .alpha.-olefin polymers. While such faCtors are many and complex, a major one of instant concern is that related to sequence distribution of the monomers throughout the polymer chain.
For polyolefin plastics, sequence distribution is of little consequence in determining polymer properties since primarily one monomer is present in the chain. Accordingly, in plastic copolymers the majority monomer will be present in the form of long monomeric blocks.
While sequence distribution is thus of little concern with respect to polymeric plastics, it is a critical factor to be considered with respect to elastomers. If the olefinic monomers tend to form long blocks which can crystallize, elastic properties of the polymer are poorer than in a polymer with short monomer sequences in the chain.
For example, titanium catalysts which can produce stereoregular propylene sequences ar particularly disadvantageous since creating blocks of either ethylene or propylene will lead to undesirable levels of crystallinity in an elastomer.
At a given comonomer composition, sequence distribution is primarily a function of the catalyst components chosen. It can thus be seen that the artisan must exercise extreme care in selecting a catalyst system for making elastomers, with their critical dependency on sequence distribution. It can also be seen that, on the other hand, no such restrictions apply to the selection of a catalyst system for making plastic polymer.
To avoid crystallinity in copolymers, it is also necessary to use a catalyst that produces a material with a narrow compositional distribution so that fractions containing a high content of one monomer are not present.
Furthermore, when making ethylene-.alpha.-olefin copolymers it is well known that the .alpha.-olefin may act as a chain transfer agent. For essentially crystalline copolymers with low .alpha.-olefin content, the molecular weight modifying effect of the .alpha.-olefin may be insignificant. However, when making copolymers with compositions in the elastomer range, catalysts that give high molecular weight plastic copolymers may produce low molecular weight polymers unsuitable for elastomer applications. In a similar fashion, undesirable molecular weight distribution changes can occur or the compositional distribution can change.
In view of the complicated and poorly understood relationship between polymer composition and catalyst performance, it is difficult for the artisan to predict the behavior of a catalyst for the production of an elastomer if it has only been used previously to make plastic homo- or copolymers.
European Patent Application 206,794 discloses that certain supported metallocene/alumoxane systems, particularly bis(cyclopentadienyl) transition metal metallocenes, in which the cyclopentadienyl ligands are unsubstituted or substituted and may be bridged by an alkylene or a silanylene group, are useful for polymerizing ethylene to a homopolymer or to a copolymer with an .alpha.-olefin for purposes of modifying the clarity or impact properties of the polyethylene polymer product.
The art has also indicated that amorphous ethylene-propylene copolymers may be produced by metallocene/alumoxane catalyst systems in which the metallocene component is a particular species of metallocene. As used in the art the term "EPC" means a copolymer ethylene and an .alpha.-olefin which exhibits the properties of an elastomer as defined in ASTM D1566 under rubber. However, as heretofore reported, the EPC's so produced have been too low in molecular weight to be suitable for use as a commercial elastomeric material, especially when the elastomer has more than 20 wt % incorporated propylene. Also, the activities of the catalysts employed have been too low for production of products with low residues of catalyst in a reasonable time.
In U.S. Pat. No. 4,937,299 of Ewen et al, issued June 26, 1990 it is indicated that an alumoxane system with dimethylsilanylenedicyclopentadienyl zirconium dichloride or bis(cyclopentadienyl) titanium diphenyl will catalyze production of a low molecular weight EPC, and that such catalyst systems may be employed in conjunction with other distinct metallocene/alumoxane catalyst systems to produce reactor blends of an EPC with high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) such as, HDPE/EPC, LLDPE/EPC, HDPE/LLDPE/EPC reactor blends or the like. The EPC component of the blends so produced--which by itself by reason of its low molecular weight is not a commercially useful elastomer--is useful in the context of a modifier blend component for the base HDPE or LLDPE with which it is coproduced.
Japanese Kokai numbers 119,215; 121,707; and 121,709 disclose production of soft copolymers variously of ethylene-.alpha.-olefin, propylene-.alpha.-olefin, butylene-a-olefin, using a metallocene/alumoxane catalyst system wherein the metallocene is a metal salt of a lower alkylene bridged -bis(cyclopentadienyl), -bis(indenyl) or -bis(tetrahydroindenyl) compound. The Japanese Kokai represent that copolymer products may be produced by a gas or liquid phase reaction procedure to have a wide range of properties such as crystallinities from 0.5-60%, while having a molecular weight distribution (MWD) less than 3 with low levels of boiling methyl acetate soluble components. The Japanese Kokai represent that such copolymerization may be carried out in the presence of such catalysts at temperatures from -80.degree. to 50.degree. C. under pressures ranging from ambient to 30 kg/cm.sup.2. Yet in the examples of the first two Japanese Kokai, which illustrate actual production of such materials, the reaction conditions illustrated are temperatures of -10.degree. to -20.degree. C. at reaction times of from 5 to 30 hours using solution polymerization with toluene as the solvent. A process as illustrated by the operating examples of the first two Japanese Kokai is not attractive from a standpoint of commercial production since the long reaction times, low temperatures and need to separate polymer product from the reaction solvent impose severe increases in production cost of the resulting copolymer material. The process of Japanese Kokai 121,709 is also unattractive for commercial production due to the use of toluene as a solvent and the expense of separating and recycling the large volume of solvent.
A process by which EPC elastomers of commercially acceptable properties may be produced within a range of reaction conditions which are suitable for commercial practice, using metallocene/alumoxane catalyst systems, has not been demonstrated. For an EPC elastomer to be considered to have commercially acceptable properties, it should have a Mooney viscosity (ML.sub.1+8 at 127.degree. C.) no less than 10, a weight-average molecular weight no less than 110,000, a glass transition temperature below -40.degree. C. and a degree of crystallinity no greater than 25%. For certain applications, e.g. extrusion performance, EPC elastomers should also have a molecular weight distribution characterized by the ratio of weight-average to number-average molecular weights of 5 or less. The range of reaction conditions most economical, hence commercially viable for practice, under which EPC elastomers should be produced is a reaction temperature ranging from 0.degree. to 80.degree. C. at reaction residence times of less than 6 hours. Desirably, the reaction conditions should minimize or eliminate the number of extrinsic treatment steps needed to isolate the polymer product in final marketable form. Hence, it is desirable for the production method to employ as a reaction diluent one or more of the monomers rather than an inert solvent from which the polymer product must later be separated. It is also desirable that the product be produced in the form of granules suspended in the reaction medium for ease of isolation and subsequent processing. Finally, it is desirable that the catalyst be sufficiently active that removal of catalyst residue (deashing) from the product is not needed.